The accelerated growth of the scope and requirements governing non-destructive testing and inspection of materials has prompted a need for improved inspection techniques. As an example, the tightening of requirements governing preservice and inservice examination of nuclear power plants and components has required the advance of non-destructive inspection technology prior to its becoming a useful, practical tool for assuring reactor safety. One of the major problems in nuclear safety is the detection and location of growing flaws such as fatigue cracks in components of the system. Environmental problems such as high temperature and radiation are some of the operational characteristics which must be contended with during the inspection of the reactors. The unique properties of materials utilized in the manufacture of reactor system components also present special problems when attempting to use conventional, non-destructive inspection techniques.
Some coarse-grained austenitic materials, such as static and centrifugally cast stainless steel, are used extensively in the design and construction of nuclear power plant systems and components. Austenitic materials provide metallurgical and economical advantages when used in construction of these systems. However, the material properties of these steels present formidable ultrasonic inspection problems, especially in heavy section vessels and piping components. Characteristics of the material, which contribute to high values of ultrasonic attenuation include grain structure; grain boundary precipitates; grain boundary orientation; and intergranular constituents resulting from preferential or nonuniform cooling rates.
As an example, metallurigical evaluations of the grain structure associated with both static and centrifugally cast stainless steel pipe show the microstructure has a thin layer of equiaxed grains located near the inner diameter of the pipe yielding to a well-defined layer of large columnar grains progressing to the outer diameter of the pipe. This type of grain matrix coupled with a high degree of grain boundary precipitation yields a material structure exhibiting exceptionally high ultrasonic attenuation to a shear mode of particle vibration, and causes the material to exhibit anisotropic behavior from an overall ultrasonic viewpoint.
Material property conditions of this nature, when combined with nonuniform surface conditions normally associated with cast components, generally preclude successful ultrasonic inspection with conventional techniques. Thus, conventional ultrasonic inspection techniques employing either a shear (transverse) mode or longitudinal (compressed) mode of particle vibration have not provided the sensitivity, resolution, reliability, and capability to inspect the types of materials discussed to the degree required by the ASME Boiler and Pressure Vessel Code. In the case of the shear wave the referred to ultrasonic attenuation is the primary reason for lack of positive results. However, failure to successfully inspect such materials with longitudinal ultrasonic waves has been as the result of the failure by the prior art to recognize certain critical factors as described herein.
It is thus an object of this invention to provide highly improved inspection by ultrasonic methods and apparatus of materials such as austenitic steel, whose physical characteristics are generally highly attenuative to ultrasonic waves.
Another object of this invention is to provide ultrasonic inspection apparatus particularly adapted to provide for such improved inspection, and methods of utilizing such apparatus.
Another object of this invention is to provide such apparatus which is relatively simple and inexpensive and can be adapted to use the methods of this invention to inspect objects of different sizes and configurations.
These and other objects of this invention, which will become apparent upon consideration of the appended drawings and claims, and the detailed description of the preferred embodiments of this invention to follow, are accomplished according to this invention by employing a longitudinal wave, i.e., a compressional mode of particle vibration within the material, under controlled and critical conditions to accomplish ultrasonic inspection. As a result of tests conducted by the inventor hereof, this mode of particle vibration has proven to be quite effective when employed under the conditions set out herein in penetrating coarse-grained austenitic steels as well as other types of materials such as ferrous alloyed steel.
In utilizing the longitudinal wave for inspection in accordance with this invention, the process of launching the ultrasonic wave into the material employs several critical techniques. Specially designed wedges, fabricated from plastic material such as Lucite, introduce the longitudinal wave into the material at a prescribed angle depending upon the depth where inspection is to be accomplished or on the orientation of suspected or specific flaws. The system is designed to normally utilize two wedges, one to couple the transmitting transducer which generates the ultrasonic wave and the second to couple the receiving transducer which detects the wave reflected from internal reflectors within the material. By using two independent wedges, the transmitter and receiver networks can be physically isolated to prevent "cross talk" between them, and the wedges may be positioned selectively on the material to be inspected, depending on the specific application.
The wedges also provide for an included angle between the transmitted and reflected wave. This angle is critical and must be adjusted for inspection at various depths in the material as hereinafter explained. Also, the linear separation between the transmitting and receiving wedges is critical, and this distance must be selected to provide proper geometry for the reflected wave to be received by the receiving wedge, but not small enough to allow substantial ultrasonic noise to couple between the two wedges. The transmitting transducer mounted on the wedge is selected to provide the highest efficiency in generating pulsed ultrasonic waves. The sound beam is unfocused; however, normal beam spread provides for a fan-shaped sound beam of essentially equal power distribution within the material.
The ultrasonic energy concentrated within this beam dictates the area in which inspection can be accomplished. In a similar manner, the received transducer mounted upon its wedge is selected to provide the highest efficiency in detecting the wave reflected from internal reflectors. By proper selection of highly efficient transmitting and receiving transducers, and proper selection of wedge angle and separation and proper positioning the ultrasonic energy can be concentrated into the region of interest. The ultrasonic energy is maintained in a longitudinal, or compressional, mode of particle vibration for all examinations, since in contrast to shear waves, the longitudinal waves are influenced less by the material microstructure, thereby providing greater penetration capability.
Special attention is given in the present invention to the design of the Lucite wedge upon which the transducers are mounted. The height of the wedge controlling the distance of the transducer from the material to be inspected is designed to allow dissipation within the wedge of the near field of the wave generated by the transmitting transducer. The near field and far field of the transducer are different, evidenced by a variation in the angular distribution of the wave amplitude as a function of distance from the ultrasonic energy source. The near field is the region in which the wavefront is nonuniform and consists of a number of maxima and minima. The wavefront becomes more uniformly distributed at the point of highest maxima, which is normally termed Y , or the near field limit. Inspections must be performed in the far field of the transducer to achieve uniform results. The Lucite wedge upon which the transmitting transducer is positioned absorbs essentially all of the near field and permits the well-defined far field to be coupled into the material to be inspected. This is basically accomplished by providing a delay that is controlled by the height of the lucite wedge. Although not all of the near field is absorbed or allowed to dissipate, the amount absorbed is adequate to prevent erroneous inspection results caused by near field characteristics and for all practical purposes, the near field is absorbed in the wedge. The wedges are designed to be used with a fluid couplant material such as oil or glycerin to couple the ultrasonic energy from the wedge into the material to be inspected; however, other types of materials such as resilient, elastomeric couplants could be used.
The air space between the transmitting and the receiving wedges prevents the coupling of ultrasonic energy directly from the transmitting wedge into the receiving wedge. The output signal caused by this direct coupling is usually called "cross talk." If the wedges are in contact or if they are positioned closely enough to allow capillary action to draw couplant between them at the surface of the material being inspected, some energy will be coupled directly from one wedge to the other, thereby producing masking signals on the signal display, usually a CRT screen. Although a small amount of energy is coupled between the wedges through the couplant layer along the surface of the component being inspected when no capillary action is present, the level of these signals is low enough to be considered negligible.
In addition to absorbing essentially all of the near field and preventing cross talk, the Lucite wedge provides directional control of the ultrasonic energy and couples the energy into the material to be inspected. It prevents the channeling or focusing of the ultrasonic wave and provides a simple means of directing the ultrasonic beam into the area to be inspected. The size of the crystal used to generate the ultrasonic energy can be selected depending on the application and material thickness. For material thicknesses over two (2) inches, the crystal normally has an area in excess of 0.75 square inches. This large area, combined with the conditioning accomplished by the Lucite wedge, also provides for the ultrasonic wave to be concentrated in an area within the material to be inspected.
The angle of inclination of the Lucite wedges, which controls the angle at which the longitudinal wave enters and exits the material to be inspected, is selected by determining the depth into the material where inspection is to be accomplished. The distance between the transmitting Lucite wedge and the receiving Lucite wedge is also varied along with the angle, so that the reflected wave is detected by the receiving transducer. For example, if an inspection is to be performed at a depth of four (4) inches from the surface where the ultrasonic wave enters the material, the angle of inclusion will be selected and the receiving transducer will be positioned in such a manner that the echo resulting from a flaw in the material will return to a point in the surface of the material covered by the Lucite wedge upon which the receiving transducer is located. Flaws very near the surface will not be detected if the sizes and positions of the transducers and wedges cause the ultrasonic wave to return to the surface in an area outside of the receiving lucite wedge. However, this type of "shallow-depth" flaw may be detected by selecting alternate-sized transmitting and receiving wedges having a different included angle and by positioning the wedges closer to each other than in the former application.